Numerous heterocyclyluracils having herbicidal and/or insecticidal properties are already known (cf. JP-A 91-287 585, JP-A 93 202 031, Chem. Abstr. 116, 235 650 and Chem. Abstr. 120, 107 048). Thus, for example, 1-(3-chloro-5-trifluoromethylpyridin-2-yl)-3,6-dihydro-2,6-dioxo-4-trifluo romethyl-1(2H)-pyrimidine can be used for controlling weeds. However, at low application rates, the activity of this substance is not always satisfactory.